[time-nuts] Averaging Location for Position Hold

Miguel Gonçalves mail at miguelgoncalves.com
Tue Sep 13 22:03:14 UTC 2011


Hi Chris!

The problem with auto-survey is that it doesn't report the averaged position
in the log file. I haven't checked if the binary protocol enables access to
this but if it does it's just a matter of patching the driver IMHO. If I use
auto-survey should I continue using mode 1 in ntp.oncore? I believe that the
first time it will auto-survey but for the next start-ups it won't. Right?

PHK: you had your hands on this driver I believe. Am I saying something
stupid? :-)

Google Maps reports WGS84. Google Earth reports WGS84 and altitude is MSL
(Mean Sea Level) that is easily converted to GPS altitude using the Geoid
Correction.

Regarding Google Maps accuracy just look at the images I sent in my previous
message. The GPS that sits on the roof is giving coordinates that are 1
meter  away max from the truth.

Cheers,
Miguel

On 13 September 2011 22:03, Chris Albertson <albertson.chris at gmail.com>wrote:

> 2011/9/13 Miguel Gonçalves <mail at miguelgoncalves.com>:
> > Hi all!
> >
> > I am installing a timing GPS unit over a new location where I already
> have a
> > NMEA GPS with PPS (let's call it unit A). The NMEA GPS is logging every
> 16
> > seconds its GPGGA string.
>
> The Oncore UT+ can does it's own site survey automatically.  That's
> the best way.
>
> You can only compare the GPS location with Google if both are using
> the same "system".  The most common one today is WPS84 but you need to
> check.
>
> The problem is that the Earth is not a Sphere and different systems
> assume non-sphere shapes.  Getting this wrongs gives about the size
> error you observed, more or less.
>
> Also, can you really trust Google Earth as an authoritative source?
> I'm not sure.    An interesting test would be to go find a USGS
> benchmark or a section marker near you then enter it's location into
> Google.  See if Google hits the marker.
> --
>
> Chris Albertson
> Redondo Beach, California
>
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